You can post fish but it depends where in the world you are.
UK: Royal Mail will post fish, but they have to be send special delivery.
US: I believe you have to use specialist couriers that state live stock.
You use a poly box and bag, a single fish to a bag and then double bag them to stop leakage. Rape in scrunched news paper, and if it's looking cold use 24-48 hour heat packs.
I've never lost a fish yet in the post. One was lost for 2 days but survived it's ordeal.